FSK receiver

ABSTRACT

A signal processing apparatus ( 1 ) comprises an analogue RF receiver ( 2 ) which has a local oscillator ( 7 ). The analogue receiver ( 2 ) receives in use a first electrical signal from antenna ( 4 ) at a carrier frequency, which signal is modulated by a digitised modulating signal to provide a second electrical signal having an amplitude representing an instantaneous logic level of the digitised modulating signal. The apparatus further comprises a digital processor ( 13 ) for processing the second electrical signal and providing an output signal in response thereto. The digital processor is controlled in use by clock signals provided in response to actuation of a local oscillator of the analogue receiver.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for processing radiateddigitised radio frequency (RF) signals, and relates particularly, butnot exclusively, to an apparatus for processing radiated digitisedfrequency modulated (FM) radio frequency signals.

Radio frequency transmission of digital signals is usually achieved bymeans of frequency-shift keying (FSK), in which a radio frequency (RF)carrier signal is shifted in frequency between two closely spacedfrequencies, respectively above and below the carrier frequency,representing the 1's and 0's of digital serial bits transmitted. Areceived FSK signal is detected by processing the carrier signal bymeans of an analog receiver having a local oscillator to produce asignal at an intermediate frequency which is a frequency shifted versionof the RF signal at the carrier frequency. The signal at theintermediate frequency is subsequently fed to a limiting amplifier andthen to a detector, which provides an output signal having an amplituderepresenting the digital frequency modulating signal. The output fromthe receiver is in the form of a series of data bits, and is thenprocessed by means of a digital circuit which either provides outputanalog signals representing the original analog signal of which adigital version is transmitted, or output signals to enable variousfunctions to be carried out, such as automatic telephone dialling bysecurity apparatus, in response to the RF signal being transmitted.

Existing apparatus for processing digitised RF signals suffers from thedrawback that interference between the local oscillator of the analogreceiver and a clock generator, which is needed to operate the digitalprocessor limits the extent to which the analog receiver and digitalprocessor can operate in proximity to each other because of problemssuch as noise generation and the like. In order to minimise suchinterference between the local oscillator and clock generator, these twocomponents need to be shielded from each other by means of cumbersomeand bulky shielding arrangements, which increase the cost of apparatusand limit the extent to which the size of the apparatus can be reduced.

In existing apparatus, each track in the circuit will to some extentcarry harmonics of the processor clock frequency. Any control linesdriven by the processor will also occur on a “clock edge” and thereforeany action performed by the processor will generate radiated anddirectly injected noise.

Existing apparatus therefore has to include components for filtering outsuch radiated and directly injected noise and also to buffer such noise.Such components also add to the cost of the apparatus and limit theextent to which the size of the apparatus can be reduced.

Preferred embodiments of the present invention seek to overcome theabove disadvantages of the prior art.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an apparatus forprocessing radiated digitised radio frequency signals, the apparatuscomprising an analog receiver having at least one local oscillator,wherein the analog receiver in use receives the first electrical signalfrom an antenna at a carrier frequency, is modulated by a digitisedmodulating signal and provides a second electrical signal having anamplitude representing an instantaneous logic level of the digitisedmodulating signal; and

a digital processor for processing the second electrical signal andproviding an output signal in response thereto, wherein the digitalprocessor in use is controlled by clock signals provided in response toactuation of a local oscillator of the analog receiver.

By providing clock signals in response to actuation of a localoscillator of the analog receiver, the clock frequency can be arrangedto be an integral fraction of the local oscillator frequency. Both theclock signals and the signals from the local oscillator may be viewed asoriginating from a single source as they share a common control source.This means that the noise generated by the processor is in harmony withthe second local oscillator and therefore the effects of the noisegeneration are substantially eliminated. This provides the advantage ofminimising interference between the local oscillator of the analog andthe clock generator of the digital processor, which gives rise to thefurther advantage of improving the extent to which the analog receiverand digital processor can be located close to each other in a singledevice.

The output signal of the digital processor may be an analog version ofthe digitised modulating signal. In particular, the output signal may bean audio signal.

Alternatively, or in addition, the apparatus may be security apparatuswherein the output signal of the digital processor effects one or moretasks in response to receipt of a radiated RF signal.

The radiated radio frequency signals may be frequency modulated signals.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, by way ofexample only and not in any limitative sense, with reference to theaccompanying drawing, in which:

FIG. 1 shows a device for receiving digitised frequency modulated radiofrequency signals embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to the Figures, a signal processing apparatus 1 comprises ananalog RF receiver 2 which receives a digitised frequency modulatedinput signal at a carrier frequency along lead 3 from antenna 4. Theinput signal on lead 3 is amplified by input amplifier 5 and then inputto pin 1 of a receiver integrated circuit 6, for example an MC 3363integrated circuit manufactured by Motorola. The input amplifier 5 istuned to 434 MHZ.

The integrated circuit 6 includes a first mixer in which the amplifiedinput signal received at pin 1 is mixed with the output signal of alocal oscillator 7 tuned to 423.3 MHZ to generate a signal at anintermediate frequency of 10.7 MHZ which is a frequency shifted versionof the input signal received at pin 1 of the integrated circuit 6. Theintermediate frequency is the difference between the frequency of theamplifier 5 and the frequency of the local oscillator 7.

The output signal of the first mixer is output from pin 23 of integratedcircuit 6 and, after filtering by ceramic filter 8, is input to a secondmixer through pin 21 of in the integrated circuit 6 where it is mixedwith the output signal of a second local oscillator (not shown) drivenby a further oscillator 14. The output of oscillator 14 is input to pin6 of the integrated circuit 6. The second mixer outputs as a result anoutput signal of frequency 455 KHZ from pin 7 of integrated circuit 6.The second local oscillator is used to mix differentially down to 455KHZ for analogue decoding. This is achieved by mixing the signal fromthe first mixer with that of a 10.245 MHZ crystal controlled oscillator(i.e. the oscillator 14). The processor 13 may be arranged to operate atexactly the same frequency (10.245 MHZ as the crystal controlledoscillator 14. By injecting this signal into the second local oscillatorpin 6 of the ingrated circuit 6, any desired harmonic, for example, aninth order harmonic of the incoming crystal control frequency may beselected.

If the processor 13 is arranged to operate at a harmonic of thefrequency of the crystal controlled oscillator 14 then the signal may beused to drive an LC tuned circuit connected to the output of the crystalcontrolled oscillator 14. This output signal is then filtered by afurther ceramic filter 9 and is subsequently input to a limiteramplifier connected to pin 9 of the integrated circuit 6, the output ofwhich is then input to a detector circuit connected to pin 14 of theintegrated circuit 6. The detector circuit provides an output signalhaving an amplitude representing the frequency shifting of the frequencymodulated signal which, after passage through a frequency shift keying(FSK) comparator, provides an output signal at pin 18 of circuit 6 whichrepresents the instantaneous logic level of the digitised frequencymodulating signal received on lead 3 from antenna 4.

The signal taken from pin 18 of integrated circuit 6 is output at outputterminal 10 of the receiver 2, and the signal provided at outputterminal 10 is input to an input terminal 12 of a digital processor 13such as a micro-processor. The digital processor 13 is located on thesame circuit board as receiver 2 and is operated by means of a clockgenerator 14 which provides clock signals in response to the secondoscillator 23 of the analog receiver 2. The connection between theoutput of second oscillator 23 and clock generator 14 is arranged suchthat the operating frequency of the local oscillator 7 is always a fixedmultiple of the clock frequency of the clock generator 14.

The digital processor 13 receives input signal at input terminal 12consisting of a series of digitised data bits, and processes the data toprovide one or more output signals. The processor 13 includes a digitalto analog converter (not shown) which outputs an analog signalrepresenting the signal to be transmitted via antenna 4. In other words,the digital processor 13 outputs an analog audio signal, which is ananalog version of the digital frequency modulating signal transmittedvia antenna 4.

Alternatively, the digital processor 13 may output a digital automaticdialling signal to generate a remote alarm call in response to receiptof RF signals via antenna 4, automatically generated by securityapparatus on the like.

It will be appreciated by persons skilled in the art that the aboveembodiment has been described by way of example only, and not in anylimitative sense, and that various alterations and modifications of theinvention are possible without departure from the scope of the inventionas defined by the appended claims.

Although the present invention has been described with reference toradiation falling within the radio frequency band, it is to beunderstood that the principles can be applied to radiation in any chosenband, for example, optical or ultra violet.

What is claimed is:
 1. An apparatus for processing radiated radiofrequency signals, the apparatus comprising: (i) an analogue receiverfor receiving a radio frequency signal representing a modulating signalmodulated onto a carrier signal, the receiver including at least onemixer and a respective local oscillator connected to said at least onemixer, wherein said at least one mixer is adapted to receive arespective input signal and a respective reference signal from thecorresponding said local oscillator and to output a frequency—shiftedversion of the respective said input signal, and wherein said receiveris adapted to sequentially mix said radio frequency signal by means ofat least one said mixer, and a demodulator for receiving said radiofrequency signal subsequently to said sequential mixing thereof, and toderive said modulating signal; and (ii) a processor for processing saidmodulating signal and including a clock generator, wherein said clockgenerator and said at least one mixer are operated in use by means of acommon oscillator.
 2. An apparatus according to claim 1, wherein theanalogue receiver includes a plurality of said mixers.
 3. An apparatusaccording to claim 2, wherein the clock generator and the localoscillator of the last mixer to which said input signal is sequentiallyinput are operated in use by means of said common oscillator.
 4. Anapparatus according to claim 1, wherein the clock generator in usedrives at least one said local oscillator.
 5. An apparatus according toclaim 1, wherein the processor is a digital processor.
 6. An apparatusaccording to claim 1, wherein the modulating signal represents digitaldata transmitted by said radio frequency signal.
 7. An apparatusaccording to claim 1, further comprising an LC tuned circuit connectedto the output of said common oscillator.
 8. An apparatus according toclaim 1, wherein the processor is adapted to output an audio signal. 9.An apparatus according to claim 1, wherein the apparatus is a securityapparatus and said processor is adapted to output a signal which effectsone or more tasks in response to receipt of said radio frequency signal.10. An apparatus according to claim 1, wherein said radio frequencysignal is a frequency modulated signal.
 11. An apparatus according toclaim 1, wherein the common oscillator is a local oscillator of theanalogue receiver.